Field of the Invention
The invention relates to a circuit configuration and a method for ascertaining the temperature of a current-regulated electric coil, in particular the coil of an electromagnetic proportional valve of a hydraulic regulating device, by measurement of the ohmic resistance of the coil winding. The circuit configuration and method serve to ascertain the temperature, for instance of the coil of an electromagnetic proportional valve in a hydraulic regulating device as is used, for instance, in motor vehicle steering systems.
A rear axle steering system which is known from U.S. Pat. No. 4,770,264 includes such a regulating device with a hydraulic actuator or final control element that has a hydraulic operating cylinder and an electrically actuated control valve or proportional valve which determines the flow of fluid to the operating cylinder. It also includes an electronic control unit, by which the signals furnished by various sensors in the motor vehicle are evaluated and control signals for the final control element are generated. An electronic control unit, which is also known from Published International Application WO 89/10865, has the object of regulating the location of the operating cylinder piston in a rear axle steering system.
The oil throughput quantities inside the components of a hydraulic final control element are dependent on viscosity and therefore on temperature. The nominal throughput values are not attained, particularly at low temperatures. One consequence thereof is that the control deviation and the positional accuracy of the regulating system become worse at low temperatures. In order to take the temperature dependency of the positional regulation into account, one can measure the temperature of the final control element, which is an approximate measure for the temperature of the hydraulic oil, with its own temperature sensor. Such a sensor and the connections required for it represent an expense which is not inconsiderable.
However, it is also known in a control system for an automatic motor vehicle transmission disclosed by German Published, Non-Prosecuted Application DE 32 40 153 A1, to determine the temperature from the temperature-dependently variable electric resistance of some transmission element. However, it has been found that the accuracy of temperature ascertainment depends not only on the temperature of the coil, and therefore on the coil resistance, as well as on the supply of voltage to the coil, but also on the total electrical resistance of the coil current circuit. That resistance depends on the other resistances, such as junction resistances, line resistances, and so forth, which taken all in all cause a temperature offset that differs from one current circuit to another and falsifies the ascertained temperature value. Although it is possible to integrate a temperature sensor directly into the coil, nevertheless because of the difficulty of securing it, the requisite lines to be extended out of the coil and the requisite plug contacts, it is very expensive and also very vulnerable to malfunction.